Infrared imaging is a widely used technology and the emergency services, enforcement agencies and armed services are equipped with both night vision systems (such as night vision goggles) and thermal infrared imaging systems. Night vision systems based on image intensifier tubes are typically used in low ambient light conditions and function by amplifying the small amounts of radiation naturally available and/or supplemented by artificial light sources and reflected from surfaces. This includes near infrared radiation, by which is generally meant radiation between the visible and thermal infrared parts of the electromagnetic spectrum having a wavelength in the range 0.78 micron to 1.4 micron.
Thermal imaging, on the other hand, can be used under all lighting conditions, including when there is no ambient lighting, in extremely low light conditions or in full daylight. This technique makes use of the fact that all bodies above 0K emit electromagnetic radiation and that in the temperature range usually encountered in inhabited regions of the earth (−20° C. to +40° C.) this radiation occurs in what is called the thermal infrared. In the thermal infrared, conventional visible markings are not obvious. At the wavelength that typical thermal infrared imagers operate (either 3-5 micron or 8-12 micron) dark and light visually coloured materials tend to have the same high thermal infrared emissivity (typically in the range 0.9 to 0.97). Thus, when at the same temperature, they emit a similar intensity of electromagnetic radiation and so they appear to be the same apparent temperature as conventional retro-reflective materials used in, for example, vehicle marking liveries. Similarly, near infrared retro-reflective materials for use with image intensifier based imaging systems have high thermal infrared emissivity (0.9 to 0.97) and are also not obvious when viewed using a thermal infrared imaging system.
As a result, identification markings designed for use at visible wavelengths and/or near infrared wavelengths have little or no contrast with the background at thermal infrared wavelengths and are not generally discernible when viewed through thermal imagers. One example of a thin film retro-reflective material which is suitable for use in the near infrared, but not in the thermal infrared, is described in U.S. Pat. No. 3,758,193 to Tung. Tung discloses a layered structured including a reflective surface and a material layer which transmits infrared radiation, but substantially absorbs visible light. The layer includes a matrix film and refractive index matched organic pigment particles, more specifically a matrix film comprising alkyds, acrylics, drying oils, polyurethanes, expoxies, polystyrenes and/or fluorinated polymers, and organic pigment particles including nitroaniline, azo and/or phthalocyanine compounds. The material is demonstrated to retro-reflect in the near infrared region, but, because of the particular selection of pigments and film materials, would not be expected to reflect radiation in the thermal infrared.
It is known to provide thermal infrared markings by providing areas of high thermal infrared emittance contrast on a surface. These areas are created by minimising both the self-emittance of thermal infrared radiation and the reflected thermal infrared energy in one area, so as to create an “apparently cold” surface, while adjacent areas having high self emittance are “apparently hot”. Some parts of the sky emit little thermal infrared radiation and are apparently cold on thermal imagers, and this may be used to minimise the reflected energy component; this phenomenon is typically referred to as “cold sky reflection”.
Various identification devices exist which are based on the cold sky reflection principle. WO 2006/016094 to O'Keefe discloses an identification device for marking an article comprising a plurality of layers including a first layer arranged to be substantially absorbing at at least one visible wavelength and a second layer arranged to be substantially reflective at thermal infrared wavelengths, the first and second layers typically being arranged in a stack upon a substrate comprising a polymer film such that the second layer is disposed between the first layer and substrate layer. The second layer is able to increase the thermal infrared contrast of identification markings when the device is inclined to the horizontal at an angle in the range 0° to 40° (in other words, when the device is mounted on a horizontal or near-horizontal surface).
In WO 2006/016094, the identification device is fabricated by depositing a layer of metal on a first surface of the substrate film, and depositing upon the metal layer a colour layer having a visible colouration. The colour layer is deposited as a lacquer using a technique such as roller coating, and desirably includes dyes such as azo, xanthene and anthraquinone dyes. The thickness of the colour layer is controlled to prevent absorption of the thermal infrared radiation emitted from the reflective layer, the preferred thickness being between 0.5 and 20 micrometers (micron). The identification device may also include an additional environmental protection layer deposited upon the colour layer, an example being a polyethylene layer having a thickness of 5 to 30 micron. Accordingly, in a typical embodiment, the device comprises separate substrate, reflector, colour and protection layers.
WO 2009/112810 to O'Keefe et al discloses a sheet of thermally reflective material which can undergo cold sky reflection when oriented vertically, the sheet having a surface texture comprising a plurality of reflecting elements having thermally reflective first facets. WO 2009/112810 is an example of a so-called “directional reflector”, i.e. a reflector which, when viewed Normal or perpendicular to the plane of the sheet, reflects a thermal infrared scene from a different direction or spatial region to that Normal to the plane of the reflecting material. The surface texture is preferably a “sawtooth” texture, and the first facets preferably form an angle of less than 45 degrees with the plane of the sheet. WO 2009/112810 describes a flexible embodiment of the thermally reflective material formed by embossing a polymer film with a corrugated, micro-structured texture, sputter-coating the surface with a 100 nanometer (nm) layer of a silver/gold mixture and subsequently spray coating a 10 micron thick layer of matt green thermal infrared transparent material.
The inventors have found that thin film identification devices according to WO 2006/016094 and WO 2009/112810 have a number of important drawbacks, including problems arising from abrasion of reflector and/or colour layers, chemical and UV instability of the colour layer, and reduced performance in dirty and/or wet conditions. Although WO 2006/016094 anticipates possible problems by providing an optional environmental protection layer, the provision of such a layer can itself adversely affect the performance of the device, and, moreover, increase's device complexity.